Four decades after its introduction into total joint replacement by Charnley, UHMWPE remains the most commonly used material for the concave part of the artificial joints, e.g. for the acetabular cup of the total hip prosthesis, or the tibial plateau of the total knee prosthesis. While it has played a central role in the success and widespread use of joint replacements, UHMWPE has also been identified as a major culprit in the most common mode of their failure—aseptic loosening. Wear particles produced by articulation of the hard, convex, usually metallic, or ceramic component against the soft polymer liner accumulate in and around the joint until the concentration of particles becomes so high that, in spite of the polymer's generally excellent biocompatibility when in bulk, they initiate a biological response leading ultimately to bone loss, loosening of the joint components and dysfunction of the joint replacement.
Different methods of wear reduction have been sought, found and introduced into clinical use over the last few decades, all focused on improving the wear characteristics of the articulation pair at their interface.
On the convex, metallic, or ceramic component these efforts included reducing roughness of the surface, increasing precision of the geometry, e.g. sphericity of the femoral head, and increasing hardness of the materials. Improved wetting of the hard surface has also been identified as an important factor in general wear reduction.
These measures have been realized through:
(i) better selection of and processing of metals, both in metallurgical aspects and in machining methods employed;
(ii) use of hard coatings, added or created in situ, by e.g. oxidation;
(iii) use of bulk ceramic components.
The reduction of wear, both in laboratory testing and from in vivo observations is on the order of factor two.
More recently, modifications of UHMWPE material through cross-linking have gained much attention. Cross-linking can be achieved by physical (irradiation) and/or chemical means. Laboratory testing has shown great variability, mostly due to different methods of wear production and assessment employed. Tests conducted on joint simulators and with careful compensation for artifacts, suggest factor five to ten reduction of wear vs. regular UHMWPE.
A number of potential problems of cross-linking have been mentioned in, by now, vast literature on the subject. Among those is the reduction of strength, particularly in fatigue; reduction of average particle size, making the wear debris biologically more active; and risk of long term degradation in the body.
Cross-linked UHMWPE is in broad clinical use for about five years—too short a time for the final judgment on its benefit-to-risk ratio. Several clinical observations suggest the actual wear reduction of about factor two, but again, there is much variability in methods used to assess the wear and thus in the reported results.
Metal-metal articulations have been used before UHMWPE was introduced in the sixties, which has ever since dominated the clinical use. As the biological problems of UHMWPE wear surfaced in the eighties, metal-metal was re-introduced, with now better metallurgical and manufacturing technologies available and consequently promising the better clinical outcomes. The wear rates, compared to UHMWPE, are on the order of factor ten lower by weight; hundred times by volume. However, systemic accumulation of ions of potentially harmful metals has been observed, and the risks remain unknown, especially in younger patients, who are most in need of improved joint replacements.
Ceramic-ceramic articulations are technically the best in terms of wear, but various regulatory obstacles and the high price have, until recently, kept their use to a small percentage of the total. There are also risks, if relatively low, of component breakage due to extreme brittleness of ceramics and rapid degradation of the articulation due to even minuscule imperfections of, or any damage to the surfaces. Sophisticated technologies and quality controls required in production have also been an impediment to their wider use. This invention offers a potential of wear reduction principally and consequently increased robustness of ceramic-ceramic articulations.